If you have ever been to Teotihuacan, Mexico, you are well acquainted with the many vendors that call the ancient city of pyramids their office. Some more dedicated than others, brave the hot sun as they circle the dusty plazas in a peacock-like display of their goods. Others linger like sleepy dogs in the shade, springing to life at the approach of an unsuspecting tourist. Whatever the species of vendor, they all share...